1. Field of the Invention
The present invention relates generally to communications networks in which cross-connect nodes are organized into a plurality of groups corresponding to geographic domains and in which constraints are imposed on intra-domain node-to-node connectivity. More specifically, the present invention relates to a path finding technique for avoiding unreachable nodes which would otherwise be encountered due to connectivity constraints.
2. Description of the Related Art
In an optical communications network, a number of optical cross-connect nodes are interconnected by optical links and wavelength division multiplexers are provided between neighbor nodes to support a number of parallel wavelength channels. Different network providers and administrators organize the optical communications network into a number of groups corresponding to geographic domains for efficient network management and administration. For establishing a path across the network, use is made of a distance vector algorithm known as the BGP (border gateway protocol) routing protocol (RFC 1771) that operates on TCP/IP. According to the BGP routing protocol, neighbor domain nodes use the BGP open message and the BGP keepalives message during a neighbor discovery process and create a routing table from BGP update messages advertised from neighbor domains. Based on the routing table, each domain perform route calculations and then advertise its calculated routes and updates its routing table. As the advertisement process is repeated, the contents of each routing table tend to converge to a set of invariable values.
On the other hand, the optical cross-connect node has the ability to perform its switching function on optical signals of any transmission rate or any data format. However, if signals are “transparently” propagated through a large number of optical nodes or transmitted over long distances, they would suffer from serious distortion due to noise and attenuation with a result that their bit error rate becomes higher than the prescribed acceptable level. Additionally, optical cross-connect nodes may also be configured with an optical add-drop multiplexer (OADM) which is only capable of performing its add-drop function on a particular wavelength. Such cross-connect nodes do not have non-blocking feature. Hence, connectivity may be constrained within a domain to such an extent that no accessibility exists between particular nodes within that domain. Connectivity constraint may arise on a particular intra-domain route. Due to this intra-domain connectivity constraint, attempts to set up a path using the BGP routing protocol may encounter a failure.
One solution is to have all network nodes share connectivity constraints information in common. However, the amount of such information each network node could hold in its memory would be significantly large, which could lead to the loss of network scalability.